Like any fuel, hydrogen stores large amounts of energy, and handling hydrogen requires safety precautions. As the use of hydrogen fuel becomes more common, there will be an increased need for reliable hydrogen sensors. Hydrogen is now used in the transportation, petrochemical, food processing, microchip, and spacecraft industries. Each of these industries needs reliable hydrogen sensors for many applications, for example, pinpointing leaks to prevent the possibility of explosions in production equipment, transport tanks, and storage tanks. Advances in fuel cell technology will provide numerous future applications for hydrogen sensors.
Hydrogen sensors, in some instances, could be used to warn of an imminent equipment failure. Electrical transformers and other electrical equipment are often filled with insulating oil to provide electrical insulation between energized parts. The presence of hydrogen in the insulating oil can indicate a failure or potential explosion. Hydrogen sensors could be utilized both under the insulating oil and in the air immediately above the insulating oil. Therefore, closely monitoring hydrogen levels in and around equipment containing insulating oil could be an effective tool in predicting and preventing equipment failure.
As fuel cell technology advances, fuel cells will see greater use as power sources for both vehicles and homes. Since hydrogen can be a highly explosive gas, each fuel cell system needs hydrogen detectors to sense and alarm in the event of a hydrogen leak. Hydrogen detectors can also be placed inside a fuel cell to monitor the health of the fuel cell. Hydrogen sensor packages are also needed to monitor hydrogen concentration in the feed gas to fuel cells for process control.
Hydrogen sensor packages in fuel cells require high sensitivity. Such sensor packages should have a wide measurement range spanning from below 1% up to 100% hydrogen. The measurement range is dependent on which fuel cell technology is used and the status of the fuel cell. Detectors are needed also to monitor for leaks in the delivery system. For transportation and other portable applications, hydrogen detectors operating in ambient air are needed to ensure the safety of hydrogen/air mixtures and to detect hydrogen leaks before they become a hazard. At high hydrogen concentration levels, issues associated with the potentially deteriorating effect on the oxygen pump operation must be addressed. Finally, hydrogen sensors must be highly selective in monitoring hydrogen in ambient air.
There are many commercially available hydrogen sensors, however, most of them are either very expensive or do not have a wide operating temperature range. Additionally, most sensors sold today have heaters included with the sensor to maintain elevated operating temperatures, requiring high power consumption that is undesirable for portable applications.
Favier et al. pioneered the use of palladium nanowires in 2001 by producing a demonstration hydrogen detector. The disclosure of Favier et al. can be read in an article published in Science, Vol. 293, Sep. 21, 2001. Hydrogen sensors prepared by this method have incredible properties due to the nature of the chemical/mechanical/electrical characteristics of the nanotechnology of palladium nanowires. The hydrogen sensors operate by measuring the conductivity of metal nanowires arrayed in parallel. In the presence of hydrogen gas, the conductivity of the metal nanowires increases.
The alpha-to-beta phase transition in the nanowire material is the mechanism for operation of these sensors. There is first a chemical absorption of hydrogen by the palladium nanocrystals of the nanowire. This causes expansion of the lattice by as much as 5-10%, causing the palladium nanocrystals that were initially isolated from each other to touch and form an excellent low-resistance wire.
However, there are many drawbacks to systems as produced and disclosed by Favier et al. A lack of complete characterization of the palladium nanowires has limited the understanding of those devices. Also, the Favier et al. method and apparatus utilizes nanowires that are electrochemically prepared by electrodepositon onto a stepped, conductive surface such as graphite. This presents a problem because nanowires prepared on conductive surfaces are required to be transferred off of the conductive surface so that the conductivity of the nanowire array can be measured more readily. Such transfers of nanowires cause degradation of hydrogen sensing at higher temperatures. In summary, the major issues with pure palladium nanowires prepared on step edges of graphite are: (1) unpredictable formation of palladium nanowires; (2) narrow temperature range of operation; and (3) narrow range of sensitivity to hydrogen concentration.
As a result, there is a need in the art for an apparatus and method for (1) predictably forming palladium and palladium alloy nanowires; (2) increasing the temperature operating range of sensors; and (3) increasing the range of hydrogen concentrations that can be measured.